1. Field of the Invention
This invention relates to a fixing belt and a fuser using the fixing belt for fixing a toner image formed in an image formation apparatus such as a printer or a copier using electrophotography on a record sheet and in particular to a fuser for heating and pressurizing the record sheet to fix a toner image and then cooling the record sheet by cooling means with the record sheet abutted against an endless belt before peeling off the record sheet.
2. Description of the Related Art
In recent years, a fuser comprising an endless belt placed on at least a heating roll and a peeling roll spaced from the heating roll and rotating, a pressurizing roll for pressing the endless belt against the heating roll, and a cooling structure for coming in contact with the inner peripheral surface of the endless belt from the heating roll to the peeling roll and cooling wherein the endless belt is rotated in a passage direction in the order of the heating roll, the cooling structure, and the peeling roll, a record sheet supporting a toner image is introduced into a press area (nip part) between the endless belt placed on the heating roil and the pressurizing roll, the portion of the record sheet with which the cooling structure comes in contact in a state in which the record sheet is abutted against the endless belt is passed through, and the record sheet is transported to the portion of the endless belt placed on the peeling roll and is peeled off, whereby the toner image is fixed onto the record sheet has been proposed as a fuser used with an image formation apparatus using electrophotography (for example, JP-A-4-216580, JP-A-5-72926, etc.,).
The fuser described in JP-A-4-216580 adopts as the cooling structure, a cooling roll of air cooling type disposed so that the cooling roll can come in and out of contact with the inner peripheral surface of the endless belt (refer to FIG. 5, etc., in JP-A-4-216580) and the fuser described in JP-A-5-72926 adopts as the cooling structure, an air cooling box comprising a large number of air ventilation holes made in a contact face with the inner peripheral surface of the endless belt (refer to FIG. 2, etc., in JP-A-5-72926) In both fusers, the record sheet supporting the toner image is heated and pressurized and then is cooled by the cooling roll or the air cooling box with the record sheet abutted against the endless belt before the record sheet is peeled off the endless belt for fixing the toner image.
By the way, in such a fuser for heating and pressurizing the record sheet to fix the toner image and then cooling the record sheet with the record sheet abutted against the endless belt and peeling the record sheet, how the cooling is executed uniformly and stably is one problem.
That is, in the fuser adopting the cooling roll, the cooling roll is simply brought into contact with the endless belt and thus the endless belt comes in insufficient contact with the cooling roll and an uncooled portion occurs because of deformation like wrinkles or waves occurring on the endless belt and consequently the toner image on the record sheet after being heated and pressurized is not uniformly cooled and it is feared that unevenness may also occur in the image quality accordingly.
In the fuser adopting the air cooling box, the air cooling box is pressed against the endless belt and thus can be brought into almost sufficient contact with the endless belt for uniformly cooling the endless belt except that the contact is impaired as much as the presence of a large number of air ventilation holes. However, if the amount of pressing the air cooling box against the endless belt is too large and the belt is bent largely, the record sheet after being heated and pressurized easily peels off the endless belt portion against which the record sheet is pressed by the air cooling box, or also easily peels off the endless belt when it leaves the endless belt portion against which the record sheet is pressed and consequently the toner image on the record sheet after being heated and pressurized is not uniformly cooled and it is feared that an image quality failure may occur because of such cooling unevenness. Moreover, although such cooling unevenness is prevented to some extent if on the opposite side of the air cooling box with the endless belt between, an endless belt with an air cooling box coming in contact with the outer peripheral surface of the endless belt is also disposed, as disclosed in JP-A-5-72926, a condition in which the record sheet easily peels off the belt always exists depending on the pressing amount of the air cooling box cooling the inner peripheral surface of the belt and thus it is feared that cooling unevenness may be induced.
Incidentally, as fixing belts, there has been known an image fixing film described in, for example, JP-A-10-111613, a fixing belt described in JP-A-11-143279, and the like have been known as fixing belts.
The former fixing belt comprises a rubber elastic layer (JIS-A hardness 1-70xc2x0, layer thickness 0.1-3 mm) made of silicone rubber, etc., and a release property surface layer (layer thickness 5-50 xcexcm) made of fluorine resin placed in order on a base material of a polyimide film. The latter fixing belt comprises a heat-resistant elastic layer (layer thickness 0.07 mm or more) made of fluorine rubber, silicone rubber, etc., on a base material of metal, etc., and comprises an outermost layer made of fluorine resin having a surface coarseness of Ra 1 xcexcm or less (layer thickness 2-100 xcexcm). Moreover, splanchnic release oil (fluorosilicone oil) is applied to the belt surface for use.
By the way, if such a fixing belt in the related art is applied to a fuser of the type wherein a record sheet P formed on the surface of a base material 1140 with a transparent resin layer 1150 made of a thermoplastic resin, etc., for supporting toner T and a fixing belt 1200 are heated and pressurized in a state in which they are overlaid on each other so that the toner T and belt surface 1200a face each other, whereby the toner T is fixed into the transparent resin layer 1150 of the record sheet P as shown in FIG. 29A, the following problem is involved:
In the fuser, as illustrated in FIG. 29B, the toner T needs to be sufficiently buried into the thermoplastic transparent resin layer 1150 and thus the record sheet P supporting the toner T and the fixing belt 1200 are heated and pressurized in a state in which they are overlaid on each other as described above, whereby the toner T and the transparent resin layer 1150 are fused and the toner T is buried into the fused transparent resin layer 1150 through the fixing belt 1200.
If the fixing is executed, an air bubble pool (so-called edge void) 1300 occurs in an edge part of an image made of the toner T (particularly a cross part of line drawings crossing each other) as illustrated in FIG. 30A; this is a problem. Such an image edge part void easily occurs if a hard fixing belt 1200 (formed with a hard resin coat layer). The possible reason is as follows: As shown in FIG. 31A, the hard fixing belt 1200 cannot sufficiently follow level difference h between an image portion of the toner T and a non-image portion (exposure face of the transparent resin layer 1150) on the record sheet P and cannot become deformed and thus a gap k is formed between the fixing belt 1200 and the transparent resin layer 1150 and pressurizing through the fixing belt 1200 at the fixing time (hollow arrows in the figure) becomes non-uniform in the presence of the gap k (namely, high pressurization state for the high-level image portion and low pressurization state for the low-level non-image portion). Consequently, the toner T is strongly and rapidly buried into the transparent resin layer 1150 by the fixing belt 1200 and thus air bubbles are involved in the boundary portion between the toner T and the resin layer, are not sufficiently lost, are cooled and hardened, and remain in the boundary portion.
If the fixing is executed, smoothing of the image surface after the fixing becomes insufficient and the image having a sense of asperities (relief-toned image) results; this is also a problem. Such an image surface smoothing failure easily occurs if a too soft fixing belt 1200 (coated with a soft resin layer). The possible reason is as follows: As shown in FIG. 31B, if there is a multiple toner image portion (pile height) with a plurality of color toners superposed like a color image, the soft fixing belt 1200 becomes deformed following the level difference between the multiple toner image portion and a non-image portion, but the pressure through the fixing belt 1200 at the fixing time is scattered (namely, the pressure concentrates on the lower-level non-image portion of a relatively wider area than the image portion) and consequently the toner T is not sufficiently buried into the transparent resin layer 1150 by the fixing belt 1200.
In addition, with a fixing belt formed with a soft coat layer made of silicone rubber, etc., if a wax component as a mold release agent is added to the toner T, the wax component is transferred (offset) to the surface of the fixing belt at the fixing time and a ghost corresponding to the offset wax component transfer state (pattern) may occur on the later fixed image. With a fuser for transporting a record sheet to fix a toner image from a heating and pressurizing section to a peeling section with the record sheet brought into intimate contact with a fixing belt, thereby fixing the toner image on the record sheet, if the toner containing the wax component is used, the adherence of the fixing belt and the record sheet to each other is degraded in the presence of the wax component and an intimate contact failure of the record sheet with the fixing belt occurs. Consequently, smoothing of the fixed image surface becomes insufficient or the record sheet being transported peels off the fixing belt and is easily detached.
The former fixing belt described above comprises the release property surface layer made of fluorine resin, so that toner onto the fixing belt or wax component offset becomes hard to occur, but the smoothness of t he release property surface layer is low and a fixed image rich in gloss, for example, is hard to provide. Further, with the latter fixing belt described above, unevenness occurs in smoothness because of the release oil applied to the belt surface and still a fixed image rich in gloss is hard to provide.
It is therefore an object of the invention to provide a fuser that can uniformly and stably cool an endless belt from a heating roll to a peeling roll and a record sheet to support a toner image to be fixed, transported with the record sheet abutted against the endless belt by cooling means for cooling while pressing the endless belt from the inner peripheral surface thereof and can accomplish good fixing with no cooling unevenness as a fuser of the type wherein a record sheet to support a toner image to be fixed is cooled by cooling means with the record sheet abutted against an endless belt and then is peeled.
It is another object of the invention to provide a fixing belt capable of executing good fixing excellent in smoothness without occurrence of voids in image edge parts or a smoothing failure of the image surface if fixing as described above is executed and a fuser capable of accomplishing such good fixing using the fixing belt.
To achieve the above objects, according to a first aspect of the invention, there is provided a fuser comprising:
a heating roll;
a peeling roll being spaced from said heating roll;
an endless belt being at least placed on said peeling roll and said heating roll and run;
a pressurizing roll for pressing said endless belt against said heating roll; and
a cooling structure having a press cooling face being disposed on an inner peripheral surface of said endless belt for cooling the inner peripheral surface portion of said endless belt from said heating roll to said peeling roll while pressing the inner peripheral surface portion in a direction of an outer peripheral surface of said endless belt;
wherein said endless belt is run in a passage direction in the order of said heating roll, said cooling structure, and said peeling roll; and
wherein a record sheet to support a toner image is introduced into a press area between said endless belt placed on the heating roll and said pressurizing roll, the portion of the record sheet with which the press cooling face of the cooling structure comes in contact in a state in which the record sheet is abutted against said endless belt is passed through, and the record sheet is transported to the portion of said endless belt placed on said peeling roll and is peeled off, whereby the toner image is fixed onto the record sheet.
In the fuser, said cooling structure is disposed so that a bend angle (xcex1) of a part bent during the time between said endless belt coming in contact with the press cooling face of said cooling structure and exiting from the press cooling face is placed in the range of 0xc2x0 C. less than xcex1xe2x89xa67xc2x0 C.
Also, according to a second aspect of the invention, there is provided a fixing belt shaped in an endless belt being over laid on a record sheet to support toner for fixing the toner onto the record sheet as said fixing belt and the record sheet are heated and pressurized, wherein minute hardness of a surface of said fixing belt coming in contact with the toner is 0.1 to 5.
Further, according to a third aspect of the invention, there is provided a fuser comprising:
a fixing belt shaped in an endless belt being overlaid on a record sheet to support toner for fixing the toner onto the record sheet as said fixing belt and the record sheet are heated and pressurized, wherein as the fixing belt, a fixing belt as claimed in claim 10 is used.